This invention is directed generally to improvements in television receivers. It is particularly directed to an improved system for developing sync pulses whose frequency is twice the frequency of conventionally broadcast sync pulses.
In many television receivers, synchronization of vertical scan is achieved by developing "2H" pulses whose frequency is twice the normal (15.73 kilohertz) horizontal scan frequency, and by counting down or dividing the 2H pulses by a constant factor to develop vertical rate pulses. To drive the receiver's horizontal scan at the nominal 1H rate, the 2H pulses may merely be divided by a factor of two.
To develop the 2H pulses, it is conventional to modify the receiver's color oscillator to operate at a frequency of four times fc, where fc is the frequency (approximately 3.58 megahertz) of the color subcarrier. A counter then divides the 4fc signal by a factor of 455 to develop the 2H pulses.
Although the conventional approach to developing the 4fc signal results in satisfactory operation, it does have certain drawbacks. For example, a crystal capable of oscillating at 4fc is usually required. Because such crystals are non-standard in the television industry, their use imposes a cost penalty.
Another drawback results from the fact that the use of a 4fc signal requires the countdown circuits to operate at a frequency in excess of 14 megahertz. Since operation at such a high frequency pushes the limits of presently available technology, the yield of such countdown circuitry is reduced.
Despite the previously mentioned problems, the 4fc signal continues to be used to develop the 2H pulses because the frequency of the 4fc signal is related to the frequency of the 2H pulses by an integer (455). This relationship makes the counting down process relatively straight forward. To count down to 2H from the nominal fc signal would overcome the foregoing problems, but it requires division by a non-integer, a process which digital circuitry has heretofore been unable to achieve effectively and inexpensively. Hence, the conventional approach has continued to be used, despite its attendant problems.